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Abstract:

Nanostructured Mn--Al and Mn--Al--C permanent magnets are disclosed. The
magnets have high coercivities (˜4.8 kOe and 5.2 kOe, respectively)
and high saturation magnetization values. The magnets are prepared from
cost effective and readily available elements using a novel mechanical
milling and annealing method.

Claims:

1. An intermetallic composition comprising a nanostructured manganese
aluminum alloy comprising at least about 80% of a magnetic τ phase
and having permanent magnetic properties.

2. The intermetallic composition of claim 1, wherein the manganese
aluminum alloy has a macroscopic composition of MnXAlYDo.sub.Z,
whereinDo is a dopant,X ranges from 52-58 atomic %,Y ranges from 42-48
atomic %, andZ ranges from 0 to 3 atomic %.

3. The intermetallic composition of claim 2 produced by a process
comprising the steps of:heating a mixture of metals comprising manganese
and aluminum to create a substantially homogenous solution;quenching the
homogenous solution to obtain a homogeneous solid;reheating the solid to
a temperature of 1150.degree. C. for 20 h;quenching the reheated
solid;crushing the quenched solid;milling the crushed solid for 8 h;
andannealing the milled solid at a temperature of 400.degree. C. for 10
minutes.

11. A nanostructured manganese aluminum alloy comprising at least about
80% of a magnetic τ phase and having a macroscopic composition of
MnXAlYDo.sub.Z, whereinDo is a dopant,X ranges from 52-58
atomic %,Y ranges from 42-48 atomic %, andZ ranges from 0 to 3 atomic %.

13. The nanostructured manganese aluminum alloy of claim 11, wherein the
manganese aluminum alloy has a macroscopic composition of
Mn54Al.sub.46.

14. The nanostructured manganese aluminum alloy of claim 11, wherein the
manganese aluminum alloy has a macroscopic composition of
Mn51Al46C.sub.3.

15. A method of producing an intermetallic composition comprising:heating
a mixture of metals comprising between 52-58 atomic % manganese and
between 42-48 atomic % aluminum to create a substantially homogenous
solution;quenching the homogenous solution to obtain a homogeneous
solid;reheating the solid to a temperature of 1150.degree. C. for 20
h;quenching the reheated solid;crushing the quenched solid;milling the
crushed solid for 8 h; andannealing the milled solid at a temperature of
400.degree. C. for 10 minutes.

Description:

[0003]Magnets may be broadly categorized as temporary or permanent.
Temporary (soft) magnets become magnetized or demagnetized as a direct
result of the presence or absence of an externally applied magnetic
field. Temporary magnets are used, for example, to generate electricity
and convert electrical energy into mechanical energy in motors and
actuators. Permanent (hard) magnets remain magnetized when they are
removed from an external field. Permanent magnets are used in a wide
variety of devices including motors, magnetically levitated trains, MRI
instruments, and data storage media for computerized devices.

[0004]High-performance permanent magnets, such as Sm--Co (HC=10-20
kOe) and Nd--Fe--B (HC=9-17.5 kOe), are generally intermetallic
alloys made from rare earth elements and transition metals, such as
cobalt. However, the high cost of rare earth elements and cobalt makes
the widespread use of high-performance magnets commercially impractical.
Less expensive magnets are more commonly used, but these magnets
generally have lower coercive forces, HC, i.e., their internal
magnetization is more susceptible to alteration by nearby fields. For
example, ferrites, which are predominantly iron oxides, are the cheapest
and most popular magnets, but they have both low coercive forces
(˜1600-3400 Oe) and low values of magnetization. Similarly,
aluminum-nickel-cobalt ("Alnico") alloys which contain large amounts of
nickel, cobalt and iron and small amounts of aluminum, copper and
titanium, have coercive forces in the range of 600-1400 Oe, which makes
exposure to significant demagnetizing fields undesirable.

[0005]More recently, Mn--Al--(C) alloys have been produced by mechanical
alloying processes. D. C. Crew, P. G. McCormick and R. Street, Scripta
Metall. Mater., 32(3), p. 315, (1995) and T. Saito, J. Appl. Phys.,
93(10), p. 8686, (2003) have shown that adding small amounts of carbon
(e.g., about 2 atomic % or less) to certain Mn--Al alloys stabilizes the
metastable τ phase and improves magnetic properties and ductility.
Crew et al. (1995) produced Mn70Al30 weight % and
Mn70.7Al28.2C1.1 weight % alloys by consolidating ball
milled powders, annealing at 1050° C. and then quenching, after
which the materials were no longer nanocrystalline. The resulting alloys
had grain sizes of about 300-500 nm and exhibited coercivities, HC,
of 1.4 kOe and 3.4 kOe, respectively. Saito (2003), produced mechanically
alloyed Mn70Al30 weight % and Mn70Al29.5C0.5
weight % alloys that had grain sizes of about 40-60 nm and coercivities
of 250 Oe and 3.3 kOe, respectively. In this study, the low coercivities
reflected the limited formation of the magnetic r phase, which was
determined to be 10% in Mn70Al30 and 40% in
Mn70Al29.5C0.5. K. Kim, K. Sumiyama and K. Suzuki, J.
Alloys Comp., 217, p. 48, (1995), produced MnAl alloys that were ball
milled, but never annealed. The alloys displayed no hard magnetic
properties, HC=130 Oe. These Mn--Al alloys are made from relatively
inexpensive materials, but the low coercivities remain a problem.

SUMMARY

[0006]The subject matter of the present disclosure advances the art and
overcomes the problems outlined above by providing nanostructured Mn--Al
alloys and a method for their manufacture. Constituents of these alloys
may be mechanically milled and heat-treated to form permanent room
temperature magnets with high coercivities and relatively high saturation
magnetization values.

[0007]In one embodiment, an intermetallic composition includes a
nanostructured manganese aluminum alloy having at least about 80% of a
magnetic phase and permanent magnetic properties at room temperature.

[0008]In one embodiment, a nanostructured manganese aluminum alloy
includes at least about 80% of a magnetic τ phase and has a
macroscopic composition of MnXAlYDo.sub.Z, wherein Do is a
dopant, X ranges from 52-58 atomic %, Y ranges from 42-48 atomic %, and Z
ranges from 0 to 3 atomic %.

[0009]In one embodiment, a method of producing an intermetallic
composition includes heating a mixture of metals that contains between
52-58 atomic % manganese and between 42-48 atomic % aluminum to create a
substantially homogenous solution, quenching the homogenous solution to
obtain a homogeneous solid, reheating the solid to a temperature of
1150° C. for 20 hours, quenching the reheated solid, crushing the
quenched solid, milling the crushed solid for eight hours, and annealing
the milled solid at a temperature of 400° C. for 10 minutes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a flowchart illustrating a method of producing magnetic
alloys according to one embodiment.

[0021]FIG. 12 shows the room temperature dependence of the coercive field
on the magnetic field strength for mechanically milled and bulk
Mn54Al46 powders annealed at 400° C. for ten minutes.

[0022]FIG. 13 shows dependence of saturation magnetization on annealing
temperatures for mechanically milled and bulk samples of various
composition.

[0023]FIG. 14 shows dependence of coercivity on annealing temperatures for
mechanically milled and bulk samples of various composition.

DETAILED DESCRIPTION

[0024]Methods for producing mechanically milled, nanostructured Mn--Al and
Mn--Al--C alloys will now be shown and described. High room temperature
coercivities and saturation magnetization values have been achieved for
Mn--Al alloys that are produced by the presently described methods, and
it has been shown that the addition of small amounts of carbon (e.g.,
about 3 atomic % or less) to Mn--Al alloys stabilizes the metastable
τ phase and improves magnetic properties.

[0025]Mechanically milled Mn--Al alloys possessing a L10-structured
magnetic τ phase, with HC=4.8 kOe and MS=87 emu/g at room
temperature, were obtained by annealing Mn54Al46 powders at
400° C. for 10 minutes. The coercivity value of this alloy is the
highest ever reported for Mn--Al materials. The amount of magnetic τ
phase present in the annealed product is estimated from the saturation
magnetization (MS of pure τ phase is ˜110 emu/g) to be
about 80%. In another embodiment, a Mn--Al--C alloy,
Mn51Al46C.sub.3, prepared by the same method displayed a
coercivity that is the highest ever reported for Mn--Al--C materials,
HC=5.2 kOe.

[0026]The macroscopic formulas presented herein, e.g., Mn54Al46,
pertain to the overall composition, but the materials have nanostructure
or microstructure of localized phase variation (e.g., γ, β,
and/or τ phases). As used herein, a "nanostructured" material is a
bulk solid characterized by localized variation in composition and/or
structure such that the localized variation contributes to the overall
properties of the bulk material.

[0027]The large coercive forces observed are believed to result from small
grains of the magnetic τ phase (˜30 nm) being magnetically
isolated from one another. This lack of magnetic exchange coupling may
result from non-magnetic phases (e.g., β, γ) inhibiting
changes in the alloy's internal magnetization when an external magnetic
field is applied (i.e., the non-magnetic phase(s) act as magnetic domain
wall pinning sites).

[0028]The alloys disclosed herein are resistant to corrosion and may, for
example, be used in applications currently utilizing known permanent
magnets. In one embodiment, small particles or powders of the alloys may
be produced in a resin or plastic bonded form according to known methods.
The small grain size of the alloys may provide improved ductility
relative to materials with larger grains.

[0029]FIG. 1 is a flowchart illustrating a method 100 of producing
magnetic alloys according to one embodiment. In a first step 102, a
mixture of metals, which may be in the form of ingots, powders, ribbons,
pellets or the like, is melted to provide a liquid solution. In a second
step 104, the liquid solution is quenched to form a solid solution. Steps
102 and 104 may be repeated to ensure that adequate mixing results in the
formation of a substantially homogeneous solid solution. A "substantially
homogenous" solution has a uniform structure or composition throughout,
such that in a randomized sampling of the solution at least 95% of the
samples would have consistent compositions. In step 106, the
substantially homogenous solid solution is reheated to a diffusion
temperature that is just below the melting temperature of the solid. The
solid is held at the diffusion temperature for a period of time that is
sufficient for the solid diffusion process to reach completion. For
example, the solid may be held at the diffusion temperature for twenty
hours. In step 108, the solid is quenched, e.g., with water, to halt the
diffusion process, and isolate the solid without structural rearrangement
that would otherwise occur in a slow cooling process. In steps 110 and
112, the quenched solid is crushed and milled to repeatedly fracture and
cold weld the particles in order to form a nanostructured material. The
milling is sufficient to cause a rupture of the crystals of the alloy as
well as to allow sufficient interdiffusion between the elementary
components. In step 114, the milled solid is annealed to ensure complete
formation of the nanostructured magnetic alloy.

Example 1

Production of Mn54Al46

[0030]Mn54Al46 alloy ingots were prepared by arc-melting
stoichiometrically balanced quantities of Mn and Al in a water-cooled
copper mold (Tm≈1250-1350° C.). The melted metallic
solution was then heated until molten. Quenching was performed by
allowing the alloy to rapidly cool in the copper mold to a temperature of
˜30° C. in approximately 10 minutes. Ingots were flipped and
melted a minimum of three times under argon to ensure mixing. Ingots were
subsequently heated to and held at 1150° C. for 20 h followed by
water quenching to retain the ε phase. The ingots were then
crushed and milled for eight hours in a hardened steel vial using a SPEX
8000 mill containing hardened steels balls with a ball-to-charge weight
ratio of 10:1. The vials were sealed under argon to limit oxidation. Both
the as-milled powders and the quenched bulk samples were annealed at
temperatures from 350-600° C. for 10-30 minutes to produce the
ferromagnetic L10 τ phase.

[0031]The magnetic properties were measured at a room temperature of about
20° C. using a LakeShore 7300 vibrating sample magnetometer (VSM)
under an external magnetic induction field of 15 kOe. Some samples were
also measured with an Oxford superconducting quantum interference device
(SQUID) magnetometer under a field of 50 kOe. Accuracy of the magnetic
measurements is within ±2%. Therefore, magnetic data may be reported
as "about" a particular value to account for ubiquitous sources of error
(e.g., magnetic fields within or near the magnetometer and errors
associated with weighing samples). Microstructural characterization was
performed using a Siemens D5000 diffractometer with a Cu X-ray tube and a
KeVex solid state detector set to record only Cu Kα X-rays.

[0032]FIGS. 2-5 show X-ray diffraction patterns of Mn54Al46
annealed at various temperatures. X-ray diffraction patterns for
as-milled alloys showed peaks corresponding to the h.c.p. ε phase
of the MnAl alloy. As shown in FIG. 2 the diffraction peaks were broad
and of low intensity, indicative of a nanocrystalline grain structure.
The grain size of the ε phase calculated from the (111) X-ray
peak using the Schemer formula was 8 nm. Annealing the as-milled sample
of Mn54Al46 at 400° C. for 30 minutes caused the c phase
to transform to the f.c.t. τ phase. FIG. 3 shows peaks indicative of
the τ phase marked by asterisks. The calculated τ phase grain
size was ˜27 nm, which is much smaller than that produced by
conventional casting, grinding or extruding. Without being bound by
theory, the smaller grain size appears to result from the τ phase
forming from the nanocrystalline c phase. Increasing the annealing
temperature to 500° C. for 30 minutes caused decomposition of the
τ phase, as shown in FIG. 4 by a decrease in intensity of the τ
phase peaks. Annealing at 600° C. for 30 minutes resulted in a
minimal presence of the τ phase in the final product, as shown in
FIG. 5.

[0033]These results show that the improved magnetic performance may be
related to small grain sizes, where the nanostructured ε phase
material is transformed to the ferromagnetic τ phase at anneal
conditions characterized by the 400° C. anneal which produced the
results of FIG. 3. The effective temperature range for this anneal is
between 300° C. and 600° C., and more preferably from
350° C. to 500° C., and most preferably from 350° C.
to 450° C. The smaller grain sizes are facilitated by the milling
that occurs just prior to the anneal.

[0034]FIGS. 6 and 7 show the sensitivity or dependence of saturation
magnetization, MS, and coercivity, HC, upon annealing
temperatures for both bulk (FIG. 6) and mechanically milled (FIG. 7)
Mn54Al46. For bulk samples, the MS tends to increase with
increasing annealing temperature from 300° C. to 500° C.
The MS for mechanically milled Mn54Al46 increases from
350° C. to 400° C., then decreases with increasing
annealing temperature from 400° C. to 600° C. This is
consistent with the X-ray diffraction data (FIGS. 3-5) that showed the
volume fraction of the magnetic τ phase decreasing with annealing
temperatures above 400° C. The HC changes relatively little
from 350° C. to 500° C. for mechanically milled samples.
The optimal magnetic properties for mechanically milled samples,
HC=4.8 kOe, and MS=87 emu/g, were obtained for
Mn54Al46 powders annealed at 400° C. for 10 minutes. The
coercivity value of the mechanically milled alloy is the highest reported
to date for Mn--Al magnetically isotropic powders. In general, the
MS obtained for annealed, mechanically milled samples was lower than
that obtained in bulk samples, while the HC was higher, due to the
small τ phase grain size.

[0035]FIGS. 8 and 9 show room temperature magnetic hysteresis loops for
mechanically milled (solid squares) and bulk (open squares)
Mn54Al46 powders annealed at 400° C. for 10 minutes.
FIG. 8 shows hysteresis loops in a 15 kOe field. Coercivity is measured
as the distance along the x-axis from the origin to the intersection of
the curve with the x-axis. It can be seen that the mechanically milled
sample has a much larger coercivity (˜5 kOe) than the bulk sample
(˜1 kOe). Remanent magnetization, Mr, is the intrinsic field
of the sample when the applied field is zero. Mr of the mechanically
milled sample is approximately 35 emu/g, while that of the bulk sample is
approximately 25 emu/g. FIG. 9 shows hysteresis loops in a 50 kOe applied
field. Magnetic saturization, MS, has not been reached, as evident
from the increasing magnetization at high fields. For the mechanically
milled sample, the remanence ratio, Mr/MS, is about 0.5 when
the applied field is 50 kOe, which is characteristic of materials that
are not exchange-coupled.

[0036]FIGS. 10 and 11 show isothermal remanence magnetization (IRM), dc
demagnetization (DCD) and difference curves for mechanically milled
Mn54Al46 annealed at 400° C. for 10 minutes. FIG. 10
shows the IRM and DCD curve for the mechanically milled sample, and FIG.
11 shows the δM curves for both mechanically milled and bulk
samples annealed at 400° C. for 10 minutes. Remanence curves and
δM plots were used to determine the interaction between the
τ-phase grains. The dc demagnetization (DCD) curve shows the progress
of the irreversible changes in magnetization. The isothermal remanence
(IRM) curve contains contributions from both reversible and irreversible
magnetization processes. δM is defined as Md
(H)-[Mr(Hsat)-2Mr(H)]. A plot of δM versus H
therefore gives a curve characteristic of the interactions present. The
overall negative and small δM for the mechanically milled sample
indicates that most of the τ phase nanograins are isolated with only
small dipolar interactions between them. No exchange coupling exists in
this nanostructured material, which explains why the remanence ratio is
close to 0.5.

[0037]FIG. 12 shows the dependence of the coercive field on the magnetic
field strength for mechanically milled and bulk Mn54Al46
powders annealed at 400° C. for 10 minutes. The bulk sample curve
rises steadily to near saturation. In contrast, the mechanically milled
sample curve rises gradually at low fields until the field strength
approaches HC (5 kOe), then it rises quickly to near saturation.
This behavior indicates that the mechanism for the magnetization process
of the mechanically milled material is controlled by domain wall pinning,
and that the applied field gradually removes the domain walls from their
pinning sites. The non-magnetic phase(s) that are present could act as
the pinning sites.

Example 2

Alloy Content Sensitivity

[0038]The manufacturing process of Example 1 was repeated by varying the
content of the Mn and Al metals, and doping with carbon. FIGS. 13 and 14
show the dependence of saturation magnetization and coercivity on
annealing temperatures for mechanically milled and bulk samples of
various composition after the samples had been annealed for thirty
minutes. The legends of FIGS. 13 and 14 show Mn content, and optionally C
content, where the remainder of the sample is Al. All samples are
mechanically milled, except for those labeled "bulk". It can be seen that
1-3 atomic % carbon decreased MS but increased HC in some
cases. In particular, Mn51Al46C.sub.3 had the highest HC
observed to date for a Mn--Al--C alloy, 5.2 kOe. Dopants other than
carbon may include boron and the rare earth metals. Generally, it can be
noted that because the t phase is the only ferromagnetic phase in the
Mn--Al or Mn--Al--C systems, the saturation magnetization is proportional
to the percentage of the τ phase in the alloys. When the Mn content
is 50 atomic percent or less, little ε phase can be developed,
and therefore only a small amount of τ phase can be produced. Also,
when the Mn content is high, excess Mn is used to stabilize the
metastable τ phase. In this case, some Mn atoms occupy lattice sites
where they are coupled antiferromagnetically to other nearby Mn atoms,
thereby reducing the magnetization. Thus, the Mn content is preferably
between 52 and 58 atomic percent and the alloys may be described
according to Formula (I);

MnXAlYDo.sub.Z, (1)

[0039]wherein

[0040]Do is a dopant,

[0041]X ranges from 52-58 atomic %,

[0042]Y ranges from 42-48 atomic %, and

[0043]Z ranges from 0 to 3 atomic %.

[0044]In a more preferred sense:

[0045]Do is carbon,

[0046]X ranges from 53-56 atomic %,

[0047]Y ranges from 44-47 atomic %, and

[0048]Z ranges up to 3 atomic %.

[0049]In a most preferred sense, X is 54, Y is 46, and Do is not
necessarily present.

[0050]The above description of the specific embodiments may be modified
and/or adapted for various applications or uses that do not depart from
the general scope hereof. Therefore, such adaptations and modifications
should and are intended to be comprehended within the meaning and range
of equivalents of the disclosed embodiments. It is to be understood that
the phraseology or terminology employed herein is for the purpose of
description and not limitation.

[0051]This specification contains numerous citations to references such as
patents, patent applications, and publications. Each is hereby
incorporated by reference.

Patent applications by Ian Baker, Etna, NH US

Patent applications in class Heating or cooling of solid metal

Patent applications in all subclasses Heating or cooling of solid metal